Fix _dd.p.ksr scientific notation for very small sampling rates

ext/priority_sampling/priority_sampling.c

@@ -1,6 +1,7 @@
 #include "../compat_string.h"
 #include "priority_sampling.h"
 
+#include <math.h>
 #include <vendor/mt19937/mt19937-64.h>
 
 #include <uri_normalization/uri_normalization.h>
@@ -65,8 +66,20 @@ static void dd_update_decision_maker_tag(ddtrace_root_span_data *root_span,
 static void dd_update_knuth_sampling_rate_tag(ddtrace_root_span_data *root_span, double sample_rate) {
     zend_array *meta = ddtrace_property_array(&root_span->property_meta);
 
+    // Round to 6 decimal places at integer level to avoid IEEE 754 precision issues,
+    // then format with fixed-point notation (never scientific notation).
+    double rounded = floor(sample_rate * 1e6 + 0.5) / 1e6;
     char buf[32];
-    snprintf(buf, sizeof(buf), "%.6g", sample_rate);
+    snprintf(buf, sizeof(buf), "%.6f", rounded);
+
+    // Strip trailing zeros and optional decimal point
+    size_t len = strlen(buf);
+    while (len > 1 && buf[len - 1] == '0') {
+        buf[--len] = '\0';
+    }
+    if (len > 1 && buf[len - 1] == '.') {
+        buf[--len] = '\0';
+    }
 
     // Skip update if already set to the same value
     zval *existing = zend_hash_str_find(meta, ZEND_STRL("_dd.p.ksr"));

tests/ext/priority_sampling/028-ksr-tag-formatting.phpt

@@ -1,5 +1,5 @@
 --TEST--
-_dd.p.ksr propagated tag formats rate with up to 6 significant digits and no trailing zeros
+_dd.p.ksr propagated tag formats rate with up to 6 decimal digits and no trailing zeros
 --ENV--
 DD_TRACE_SAMPLING_RULES=[{"sample_rate": 0.7654321}]
 DD_TRACE_GENERATE_ROOT_SPAN=1

tests/ext/priority_sampling/029-ksr-tag-small-rate-formatting.phpt

@@ -0,0 +1,19 @@
+--TEST--
+_dd.p.ksr uses decimal notation (not scientific) for very small sampling rates
+--ENV--
+DD_TRACE_SAMPLING_RULES=[{"sample_rate": 0.000001}]
+DD_TRACE_GENERATE_ROOT_SPAN=1
+--FILE--
+<?php
+$root = \DDTrace\root_span();
+
+\DDTrace\get_priority_sampling();
+
+$ksr = isset($root->meta["_dd.p.ksr"]) ? $root->meta["_dd.p.ksr"] : "-";
+echo "_dd.p.ksr = ", $ksr, "\n";
+// Must be decimal notation, not scientific (e.g. "0.000001" not "1e-06")
+echo "no_sci_notation = ", (strpos($ksr, 'e') === false) ? "true" : "false", "\n";
+?>
+--EXPECT--
+_dd.p.ksr = 0.000001
+no_sci_notation = true

tests/ext/priority_sampling/030-ksr-tag-rounding-boundary.phpt

@@ -0,0 +1,19 @@
+--TEST--
+_dd.p.ksr rounds small rates correctly at 6-decimal boundary
+--ENV--
+DD_TRACE_SAMPLING_RULES=[{"sample_rate": 0.00000051}]
+DD_TRACE_GENERATE_ROOT_SPAN=1
+--FILE--
+<?php
+$root = \DDTrace\root_span();
+
+\DDTrace\get_priority_sampling();
+
+$ksr = isset($root->meta["_dd.p.ksr"]) ? $root->meta["_dd.p.ksr"] : "-";
+echo "_dd.p.ksr = ", $ksr, "\n";
+// 0.00000051 rounds up to 0.000001 at 6 decimal places
+echo "no_sci_notation = ", (strpos($ksr, 'e') === false) ? "true" : "false", "\n";
+?>
+--EXPECT--
+_dd.p.ksr = 0.000001
+no_sci_notation = true

tests/ext/priority_sampling/031-ksr-tag-exact-sixth-decimal.phpt

@@ -0,0 +1,18 @@
+--TEST--
+_dd.p.ksr preserves exact 0.000001 rate without rounding away
+--ENV--
+DD_TRACE_SAMPLING_RULES=[{"sample_rate": 0.000001}]
+DD_TRACE_GENERATE_ROOT_SPAN=1
+--FILE--
+<?php
+$root = \DDTrace\root_span();
+
+\DDTrace\get_priority_sampling();
+
+$ksr = isset($root->meta["_dd.p.ksr"]) ? $root->meta["_dd.p.ksr"] : "-";
+echo "_dd.p.ksr = ", $ksr, "\n";
+echo "no_sci_notation = ", (strpos($ksr, 'e') === false) ? "true" : "false", "\n";
+?>
+--EXPECT--
+_dd.p.ksr = 0.000001
+no_sci_notation = true

tests/ext/priority_sampling/032-ksr-tag-below-precision-rounds-to-zero.phpt

@@ -0,0 +1,19 @@
+--TEST--
+_dd.p.ksr rounds rate below 6-decimal precision to 0
+--ENV--
+DD_TRACE_SAMPLING_RULES=[{"sample_rate": 0.0000001}]
+DD_TRACE_GENERATE_ROOT_SPAN=1
+--FILE--
+<?php
+$root = \DDTrace\root_span();
+
+\DDTrace\get_priority_sampling();
+
+$ksr = isset($root->meta["_dd.p.ksr"]) ? $root->meta["_dd.p.ksr"] : "-";
+echo "_dd.p.ksr = ", $ksr, "\n";
+// 0.0000001 rounds to 0 at 6 decimal places
+echo "no_sci_notation = ", (strpos($ksr, 'e') === false) ? "true" : "false", "\n";
+?>
+--EXPECT--
+_dd.p.ksr = 0
+no_sci_notation = true